1. Field of the Invention
The present invention relates to a flame detector which promptly detects a fire upon its occurrence whether outdoors or indoors. More particularly, it is concerned with a flame detector which detects a fire upon occurrence without being influenced by a strong interfering light such as sunlight or a reflected light from an object having high reflectance.
2. Description of the Prior Art
It has heretofore been known that the spectral emissivity of flame has a peak at the wavelength of 4.3 .mu.m caused by CO.sub.2 resonance radiation, as indicated by "a" in FIG. 1. Flame detection through detection of an infrared ray of this wavelength is advantageous for the following points: (1) Sensitivity to flame is high, (2) there does not occur an erroneous detection because an artificial light such as illumination light scarcely contains a component of wavelength 4.3 .mu.m and (3) there is no erroneous detection caused by discharge sparks. In this connection, various flame detectors have already been proposed which detect the occurrence of flame through detection of a flare peculiar to flame in the range of wavelength 4.3 .mu.m or thereabouts.
Even in such type of flame detectors, however, an erroneous detection sometimes occurs when the sunlight is directly incident on a light sensing portion of each detector or when the sunlight reflected on a high reflectance material such as metal is incident thereon, and thus there is a problem in their outdoor use.
As to the sunlight, its portion remaining after subtracting the portion absorbed in air from the black-body radiation energy of about 5700K.degree. reaches the ground and provides a spectrum as indicated by "b" in FIG. 1. Here, the graphs "a" and "b" of FIG. 1 are normalized by each peak energies so it is impossible to make a direct comparison. In the sunlight spectrum, the intensity near the wavelength of 4.3 .mu.m is fairly low as compared with the peak due to the absorption of CO.sub.2. In view of this point there has been proposed a device in Japanese Patent Laid-Open Publication No. 128782/74 in which the detection of flame is performed using two wavelengths, one being a wavelength in the visible range wherein the radiation energy from flame is small and that from the sunlight is large, and the other being an infrared ray of wavelength 4.3 .mu.m or thereabouts. However, under the intense sunlight during the summer season, there sometimes is observed a radiation intensity of about the same as that of flame even in the vicinity of wavelength 4.3 .mu.m.
The device disclosed in the above laid-open publication No. 128782/74 is constructed so that a portion corresponding to a phase difference between an output signal of a first light sensing element having sensitivity to visible rays and an output signal of a second light sensing element having sensitivity to infrared rays is integrated and when the integrated value reaches a predetermined value, it is detected as a fire. However, in the case where the sunlight of high intensity is incident and is detected with only a slightly shifting of its phase from the phase of visible rays, flame occurrence is erroneously detected even if only interfering light is present. This inconvenience can be avoided by setting a high value to the predetermined judgment level for the integrated value, but this would result in deterioration of the actual fire detecting accuracy.